This application relates to an oxy-fuel burner and method of operating such burner, and specifically to an oxy-fuel burner having the ability to produce staged flames in two alternate configurations, wherein staging oxygen is introduced either above or below a fuel-rich primary flame, or simultaneously both above and below a fuel-rich primary flame, depending on furnace operating conditions and parameters.
Certain problems that persist in oxy-fuel combustion, and in particular oxy-fuel combustion in glass furnaces, have been solved by burners and methods of the present application.
First, it is difficult to generate high luminosity in an oxygen-natural gas flame. Soot formation in hydrocarbon gas flames is needed for substantial thermal radiation in the visible and near-infrared ranges of the electromagnetic spectrum. It is well known that the most commonly used hydrocarbon gaseous fuel, natural gas, which comprises principally methane and relatively minor amounts of other hydrocarbon gases and other diluent gases, has the lowest soot-forming propensity of all the major hydrocarbon gases. Hence, in practice it is relatively difficult to generate a high luminosity flame from natural gas. This inherent difficulty is compounded in oxy-fuel flames because any soot that is able to be generated, once formed, is short-lived. This is due to the exceedingly high reactivity of natural gas fuel with oxygen, particularly at the ultra-high flame temperatures generated in oxy-fuel combustion. One prior art approach to the problem has been through the use of an in-burner chamber for thermally “cracking” the natural gas by pre-mixing and igniting a fuel-rich oxy-gas mixture to form soot, then introducing the soot-containing products into a burner nozzle where they are mixed with oxygen to produce a flame that penetrates into the glass melting furnace. One inherent difficulty with this approach is that pre-mixing takes place in an internal chamber of the burner, and this exposes the burner metal to very high temperatures, while also risking internal fouling of burner passages due to soot deposition. Moreover, the flame formed by this device and/or method is difficult to adjust due to the very specific reactant composition required for cracking.
Another approach to soot formation relies on the coupled effects of maximizing interfacial area between fuel and oxygen by using a nozzle with high aspect ratio (i.e., wide-flame or flat-flame) and forming of a fuel-rich primary flame with the balance of oxygen gradually introduced or “staged” on the under-side of the flame adjacent the glass surface. See, for example, the burners of U.S. Pat. Nos. 5,575,637; 5,611,682; and 7,390,189. Data published in the '189 patent and reproduced herein in FIG. 48, show that peak underside (i.e., downward) flame radiation increases with increasing equivalence ratio of the primary flame; hence, at higher oxygen staging levels. While this approach is not burdened by the risk of premixing of fuel and oxygen, it is, for practical reasons, limited in the extent of oxygen staging that can be achieved. This is because oxygen flows between the flame boundary and internal walls of the precombustor, and thereby serves to convectively cool the precombustor walls from flame radiation and impingement. The extent of oxygen staging of the primary flame is thus dictated by cooling requirements, and the primary flame fuel-oxygen equivalence ratio is generally limited to a maximum of 3 (i.e., about ⅔ of the oxygen flowing through the secondary or staging nozzle, with ⅓ of the oxygen remaining in the primary nozzle to combust with 100% of the fuel, hence a 3:1 primary equivalence ratio) in commercial systems. But, as shown in FIG. 48, higher equivalence ratios are needed to maximize downward flame radiation to the glass surface.
Second, foam production and control can be a very significant problem in glass melting furnaces. The high moisture and oxygen concentrations produced in oxy/fuel combustion have been linked to higher amounts of secondary foam in glass melting furnaces than is typically present in air-fuel furnaces. Secondary foam is known to substantially restrict heat transfer between the combustion space and the glass melt, which lowers average glass temperature leading to poorer glass quality and decreases overall thermal efficiency, while increasing refractory temperature and erosion rates, thus shortening refractory lifetime. In the past, others have attempted to reduce or mitigate foam by various ad hoc methods with varying degrees of success. Some of these methods include; a) adding or removing refining agents to/from the batch, b) spraying fuel onto the surface of the glass, c) changing the burner oxygen/fuel or air/fuel ratio to more fuel-rich operation, d) reducing glass pull rate, e) increasing furnace pressure, and f) adjusting burner firing rates. Often, however, the same approach will not work in different furnaces, due for example to differences in batch chemistry, furnace temperature and flow patterns. What is therefore needed is a device and associated systematic method of foam reduction that is reliable, convenient, unobtrusive and inexpensive.
It is known that oxy-fuel combustion for glass melting has several benefits as compared to air-fuel combustion, such as lower capital cost, higher fuel efficiency, reduced NOx emissions, and higher glass quality. Oxygen staging can further increase those benefits. In particular, oxygen staging can be used to reduce NOx emissions and increase melting efficiency and product quality. “Oxygen staging” is a means of delaying combustion by diversion of a portion of oxygen away from the flame. Preferably, near-flame staging is used in which the staged oxygen stream (or streams) maintains a proximity to the flame to ensure eventual co-mixing and complete combustion of the fuel with oxygen.
The '189 patent describes an oxy-fuel burner with typical “under-staged” oxygen, and produces a flame that illustrates several key principles. The flame, being initially deprived of stoichiometric oxygen, generates soot and carbon monoxide (CO), the magnitude of which increases with the percentage of staged oxygen. The sooty region, in particular, comprises a cloud of microscopic carbonaceous particles and can be quite opaque, thereby presenting an impediment to radiation heat transfer. Conversely, due principally to the reaction of soot and staged oxygen, the under-side of the flame is very luminous, and transmits high rates of thermal radiation in the visible and near infrared regions of the electromagnetic spectrum. Since the radiation finds strong resistance in the adjacent soot cloud, the majority is directed downward to the glass surface. Melting efficiency is thereby increased relative to an un-staged flame. Moreover, as complete mixing of fuel and oxygen is delayed, the staged flame is longer than an un-staged one with the same fuel flow rate. This fact, combined with the enhanced visible and near-infrared radiation, ensures that peak flame temperatures are lower in the staged flame.
Results of computational fluid dynamics (CFD) modeling of highly staged and un-staged oxy-fuel flames have shown that peak temperatures of the highly staged flame are lower by approximately 600° C. The substantially lower temperature in combination with the oxygen-starved condition of the staged flame leads to lower rates of NOx generation. Photos of the burner of the '189 patent operating in both the non-staged and under-staged modes effectively illustrate the differences in flame structure and radiant properties produced by under-flame staging with oxygen (see the flame of FIG. 26A, without oxygen staging versus the flame of FIG. 26B with oxygen under-flame staging).
Replacing non-staged burners with under-staged burners has shown that under-staging the flame with oxygen increases glass bottom temperatures, and this contributes to stronger convection currents in the glass melt, promoting more complete elimination of impurities and, hence, fewer glass defects. In one typical case of a funnel glass furnace converted from non-staged to under-staged oxy-fuel burners, bottom glass temperatures increased by 10° C., while defects were reduced by nearly 50%. Furnace flue gas temperatures also decreased by 60° C., contributing to a reduction in specific fuel usage (energy input per unit output of glass) equal to nominally 9%.
However, there is another aspect to the oxygen staging/glass quality relationship that has not been addressed in previous implementations of oxy-fuel to glass furnaces—glass surface foam. Foam forms within both the batch melting (primary foam) and fining (secondary foam) processes due to the evolution of gases from the glass. Secondary foam, which consists principally of sulfur dioxide, water vapor and oxygen, is particularly prone to aggregate in a stable layer of bubbles that, at times, can grow to several inches in thickness. The principal deleterious effects of surface foam are its impedance of heat transfer to the glass, consequent reflection of thermal energy to the crown, and its corrosive properties with respect to furnace refractories. With respect to the lower rate of heat transfer to the glass, this lowers glass temperatures and weakens convection-driven secondary flows within the melt, interrupting the fining process and allowing more defects to persist through to the finished product.